Microstructural coarsening effects on redox instability and mechanical damage in solid oxide fuel cell anodes

In state-of-the-art high temperature solid oxide fuel cells (SOFCs), a porous composite of nickel and yttria stabilized zirconia (Ni/YSZ) is employed as the anode. The rapid oxidation of Ni into NiO is regarded as the main cause of the so-called reduction-oxidation (redox) instability in Ni/YSZ anodes, due to the presence of extensive bulk volume changes associated with this reaction. As a consequence, the development of internal stresses can lead to performance degradation and/or structural failure. In this study, we employ a recently developed continuum formalism to quantify the mechanical deformation behavior and evolution of internal stresses in Ni/YSZ porous anodes due to re-oxidation. In our approach, a local failure criterion is coupled to the continuum framework in order to account for the heterogeneous damage accumulation in the YSZ phase. The hallmark of our approach is the ability to track the spatial evolution of mechanical damage and capture the interaction of YSZ damaged regions with the local microstructure. Simulation results highlight the importance of the microstructure characterized by Ni to YSZ particle size ratio on the redox behavior and damage accumulation in as-synthesized SOFC anode systems. Moreover, a redox-strain-to-failure criterion is developed to quantify the degree by which coarsened anode microstructures become more susceptible to mechanical damage during re-oxidation.